Optically variable elements, such as holograms or diffraction grating images, due to their optical properties varying with the viewing angle are frequently used as protection from forgery or copy for documents of value, such as credit cards, bank notes or the like, but also for product securing on any product packagings. For the mass production of such security elements, usually, so-called “master structures” are produced, which have the respective phase information about the optically variable element in the form of a spatial relief structure. This, typically, is a glass substrate with a photoresist coating, in which the diffraction structure is preserved in the form of peaks and valleys. Starting out from this master structure, by duplicating and molding the relief structure embossing tools of any desired form are produced, with the help of which the diffraction structures reproduced by the relief structure can be transferred in large quantities to suitable substrates.
The master structure can represent the complete diffraction structure of a real hologram, or of a grating image composed of different diffraction gratings. The diffraction gratings differ from each other regarding the grating constant and/or the azimuth angle and/or the profile structure of the grating lines as well as the contour or the outline of the image area covered with the respective diffraction grating.
The grating constant corresponds to the distance between the grating lines and is of essential importance for the colour of the image area in the grating image recognizable when viewed from a certain viewing angle. The azimuth angle describes the inclination of the grating lines concerning a reference direction and is responsible for the visibility of these image fields viewed from certain viewing directions. The line profile generally is responsible for the intensity and plays a particular role in grating images of zero order. On the basis of this technique, therefore, optically variable images, e.g. moving images or also plastically appearing images, can be produced.
The individual diffraction gratings can be produced either holographically or by means of electron beam lithography. When holographically recording the diffraction gratings, in an appropriate substrate light beams consisting of spatially expanding, uniform wave fields are overlapped. For this purpose, usually, laser radiation is used. With electron beam lithography the diffracting grating lines are exposed directly to an appropriate substrate, the exposure operation frequently also being referred to as writing operation. For this method in general a glass plate is used as a substrate, which is coated with a layer sensitive to the respective particle radiation or light radiation (“photoresist”). When exposing, substrate and electron beam can be moved relative to each other. Here it is possible, to hold the substrate motionless and to electromagnetically deflect the electron beam. The deflection range of the electron beam lies within a range of a few tenths of a millimeter. In case of large-scale deflections, so-called “lens errors” of electron optics will disturb, that are noticeable also in the finished diffraction grating. Alternatively, the substrate can be moved by means of an x-y-table, while the electron beam is held motionless. For this purpose, however, a high-precision guiding of the table is required.
As to be able to produce grating images of the above-mentioned kind with the help of the electron beam lithography, the entire grating image is divided into a multitude of small fields of an edge length of up to some tenths of a millimeter. I.e. the grating image independent of the depicted motif is divided into individual “screen elements”, which by means of the electron beam are inscribed with grating lines. Here the grating lines are written into the individual small fields via the deflection of the electron beam, while the movement from field to field is effected by shifting the table. In this way large surfaces can be inscribed. This kind of electron beam exposure in general is referred to as “stitching mode”. This proceeding, however, has the disadvantage, that the image is composed of many pieces of small surfaces, which upon closer viewing are visually recognizable, coarsen the image, and lead to colour errors. In the case of larger image surfaces, such as e.g. lines, which when viewed from one viewing angle are to show a uniform colour, the surface is not provided with an appropriate, uniform diffraction grating. Instead this diffraction grating is made up of many small elements. Due to the tolerances when putting together the small surface elements, the grating lines extending across the image surface have kinks or gaps, which leads to visible errors.
In the “CPC mode” (Continuous Path Control, product of the company Leica Microsystems Ltd.), however, the electron beam is mounted stationary, while the table is moved according to the structures to be exposed. But this mode is less suitable for the production of finely structured grating images, such as for example guilloche images, or images or microwriting divided into fine lines, because these finely structured images have a predominant number of short grating lines. For that reason for each grating image a number of stop and start operations of the table has to be effected which reaches the millions. This represents a load for the table mechanism and consumes very much time.